A lithium sulfur battery has a theoretical energy density of 2,600 Wh/kg, which is far higher than a lithium ion battery (theoretical an energy density of 570 Wh/kg, a current level of around 120 Wh/kg).
However, sulfur as an active material and Li2S as a product are all nonconductors, and therefore, energy capacity is reduced during discharge.
When a reaction area of sulfur within an electrode is not sufficiently secured, the sulfur utilization rate is reduced leading to the reduction in discharge capacity, and when Li2S is locally aggregated within an anode during discharge, the resistance within an electrode increases leading to the reduction in discharge capacity. Particularly, as the loading amount of sulfur for energy density increase becomes greater, and as the current density for output improvement increases, such phenomena are intensified.
However, a high sulfur loading electrode (8 mg/cm2 or greater) is essential for storing a large amount of energy in a given space because batteries for electric vehicles (EV) need to have large absolute capacity. In addition, in order to satisfy the required output specifications, sufficient capacity (1,200 mAh/g−σ or greater) needs to be obtained without discharge voltage reduction within an electrode even at high current density.
In developing lithium sulfur batteries with such properties, designing an anode structure so as to increase a reaction area of sulfur and evenly produce Li2S all over is very important in increasing energy capacity. Sulfur according to the prior art is highly loaded in an anode of a lithium sulfur battery.
Referring to FIG. 1, anode structures for lithium sulfur batteries having high current density by including highly loaded sulfur according to the prior art may be classified into 3 types as follows.
In type 1, a volume energy density increases due to an increase of clad sheet density by rolling, and therefore, an electrode manufacturing process is simple and enables mass production.
However, cell capacity is reduced due to a narrow reaction area of sulfur, and cell capacity and lifespan characteristics are reduced due to local aggregation of Li2S. Moreover, the cell capacity and lifespan characteristics are reduced due to insufficient electrolyte replacement.
In type 2, cell capacity and lifespan characteristics are improved since electrolyte replacement is sufficient, and cell capacity is also improved since a reaction area of sulfur increases. However, the energy density is reduced since a thick and heavy carbon structure layer is used.
Lastly, in type 3, cell capacity and lifespan characteristics are improved since electrolyte replacement is sufficient, and cell capacity is also improved since the reaction area of sulfur increases. In addition, the energy density is improved compared to type 2, since type 3 is thinner than type 2. However, a volume energy density is reduced since rolling of the carbon structure layer is impossible, and the cell capacity and lifespan characteristics are reduced since Li2S is locally aggregated. Furthermore, mass production is difficult due to complicated processes.